1. Field of the Invention
The present invention relates to a semiconductor optical communication module and its manufacturing method.
2. Description of the Related Art
Optical communication systems are steadily improving in capacity and transmission rate. Systems with a 10-Gbps basic transmission rate have been implemented on a commercial basis, and next-generation systems with a 40-Gbps basic transmission rate are approaching commercialization.
These high-bit-rate high-capacity optical communication systems use electroabsorption modulated laser (EML) light sources. Compared with older systems such as directly modulated distributed feedback (DFB) laser systems, an EML optical communication system has advantages of smaller wavelength chirping and longer reach.
In consideration of the growing scale of optical communication systems, however, and for cost reasons, devices providing still longer reach are needed, e.g., forty to eighty kilometers for a 10-Gbps system operating in the 1.55-μm band, and low wavelength chirping has become essential.
To obtain low wavelength chirping, in addition to optimization of the structures of the electroabsorber (EA) and laser diode (LD) it is also necessary to suppress back-reflection at the electroabsorber end facet. If modulated light is reflected back at the electroabsorber end facet, it reenters the laser diode, causing the carrier density and hence the effective refractive index in the active layer of the laser diode to fluctuate. The resulting wavelength chirping in the laser diode, when added to the wavelength chirping in the electroabsorber, degrades the output signal waveform and results in reduced reach.
To suppress back-reflection, the use of a bent waveguide structure has been proposed. Examples of bent waveguides are shown in, for example, Japanese Patent Application Publication No. 2004-184943. In a conventional optical communication module in which the semiconductor chip including the laser diode, electroabsorber, and waveguide is aligned with the edge of the chip carrier on which the chip is mounted, a bent waveguide requires the lens assembly that receives the modulated light to be placed at angle to the chip carrier, the angle being determined by Snell's law. This angle between the chip carrier and the lens assembly has the unwanted effect of increasing the size of the optical communication module.
Another unwanted effect of the angle between the chip carrier and the lens assembly is that if moved for optical axis adjustment, the lens assembly may strike the end of the chip carrier.